3D free-assembly modular microfluidics inspired by movable type printing

Microfluidic technology has seen significant advancements, driven by microfabrication techniques like soft lithography and 3D printing. While monolithic microfluidic chips are suitable for mass production, they lack flexibility for research and development. Modular microfluidics offers a solution by allowing the assembly of individual modules with specific functions. However, existing modular approaches face challenges in material compatibility, optical transparency, and efficient small-batch production. The authors propose 3D-FAMM as a solution, drawing inspiration from the efficiency of movable type printing. This approach modularizes molds and attachments, allowing flexible construction, reusability, material selection, standardization, ease of fabrication, and cost-effectiveness. The system's versatility is demonstrated through various applications.

The paper reviews existing modular microfluidic systems, highlighting the prevalent use of soft lithography and 3D printing to create individual modules. Soft lithography, while flexible, can be inefficient for small-batch production of disposable chips. 3D printing, while promising, struggles to simultaneously achieve high resolution, surface smoothness, and the use of biocompatible and optically transparent materials. The use of PDMS cast from 3D-printed molds is mentioned as a compromise, but again suffers from inefficiencies in small batch production. Modification of injection-molded units, similar to LEGO bricks, is another approach discussed, but faces challenges in fabricating 3D structures at the micrometer scale. The authors contrast these methods with the efficiency of movable type printing, which inspired their 3D-FAMM approach.

The 3D-FAMM system consists of modular molds for microfluidic structures and modular attachments for added functionality. Molds are 3D-printed using projection micro-stereolithography, achieving feature sizes down to 15 µm. The modular size is standardized (5 × 5 mm²), simplifying assembly and interconnection. Double-layer structures are created by assembling molds in a frame and trellis, with silicone rubber bars used for compaction and stainless steel tubes for fluid connections. PDMS is cast into the assembled molds, cured, and then bonded to a glass plate. Attachments are installed in designated vacancies. Gaps between molds are filled with silicone oil to prevent flow obstruction. The methodology includes detailed descriptions of the assembly process, material selection, and dimensional characterization of the printed molds. The consistency of the mold dimensions and the effect of compression on the molds are analyzed to ensure the reusability and reliability of the system. The paper details the creation of specific microfluidic structures, including an S-shaped microchannel and a double-layer system with integrated pneumatic valves for flow control. A Christmas-tree design for concentration gradient generation is also described, including a discussion of the mixer design and the impact of hydraulic resistance on concentration profiles.

The 3D-FAMM method successfully addresses the limitations of existing modular microfluidic approaches. The standardized modules enable efficient assembly of complex 3D microfluidic systems. The use of silicone oil effectively seals gaps between the assembled molds, preventing leakage and ensuring smooth fluid flow. The double-layer design with integrated pneumatic valves allows for sophisticated flow control and creation of complex microfluidic architectures. The Christmas-tree configuration demonstrates the ability to generate both linear and non-monotonic concentration gradients by adjusting the hydraulic resistances within the network. Detailed characterization of the molds shows acceptable dimensional accuracy and surface roughness, validating the feasibility of the approach. The successful demonstration of various microfluidic applications, including concentration gradient generation, droplet-based microfluidics, cell trapping, and drug screening, highlights the versatility and potential of the 3D-FAMM platform. The methodology demonstrates high precision in the fabrication of microchannels, with fabrication errors within acceptable limits and negligible roughness. The use of silicone rubber bars ensures minimal deformation of the molds during assembly, allowing for reusability.

The 3D-FAMM platform provides a significant advancement in modular microfluidics by combining the advantages of modularity with efficient fabrication and material flexibility. The inspiration from movable type printing leads to a highly efficient and adaptable system for rapid prototyping and small-batch production. The demonstrated applications showcase the platform's versatility across diverse microfluidic functionalities. The ability to generate complex concentration gradients offers significant potential for biological and chemical applications. The standardized design and simple assembly process could facilitate wider adoption of this method within the microfluidics community. The use of biocompatible and optically transparent materials makes this suitable for various biological experiments.

The 3D-FAMM system successfully demonstrates a novel approach to modular microfluidics, offering a significant improvement over existing methods. The method's simplicity, flexibility, and cost-effectiveness make it suitable for both research and development and small-scale production. Future research directions include exploring a wider range of materials and expanding the library of modular components. Optimization of the assembly process and automation of the fabrication pipeline are also promising avenues for future development.

While the 3D-FAMM method demonstrates significant advantages, some limitations exist. The current design relies on manual assembly of the modules, which might limit scalability for large-scale production. The reliance on PDMS for casting may limit the applications in specific contexts where PDMS's properties are not ideal. Further optimization of the mold design and assembly process may be necessary for even higher precision and reproducibility.